The Department of Energy supports the development of technology that produces hydrogen by directly splitting water using highly concentrated sunlight as process heat to power thermochemical cycles. Sandia’s LSFR facility was established to characterize redox active functionality in metal oxides exposed to extreme conditions, such as high radiant heat flux and high temperature. These conditions are commonly encountered in solar powered thermochemical cycles.
Left: Schematic illustrating a generic two-step metal oxide reaction cycle. MOx is exposed to heat from concentrated solar energy causing spontaneous evolution of oxygen (left). The reduced oxide is then moved off sun and exposed to water vapor where oxygen stripped from the water molecule is reabsorbed by MOx liberating hydrogen. Right: Looking into Sandia’s radiant cavity solar receiver reactor under operation. The temperature inside the receiver is 1450 °C.
Metal oxides that can efficiently and cost effectively conduct thermochemical H2 production at industrial scale remain elusive because optimizing the hydrogen production capacity and yield in non-stoichiometric oxides has thus far proved challenging. Redox active materials must not only release oxygen at temperatures accessible to concentrating solar power reactors, but split water from their oxygen-deficient state while maintaining both high capacity and high yield. Many factors that determine a material’s behavior are deeply rooted in its electronic structure. Understanding the fundamental underpinnings that link structure to desirable thermodynamic behavior is critical to developing new materials.
Layered perovskites are not commonly used in solar thermochemical water splitting cycles. However, we have discovered a family of complex perovskites that are B-site, Mn-based compounds (Ba4AMn3O12, A=Nd, Ce, Pr) with a cerium variant that outperforms all known perovskites tested for solar thermochemical water splitting functionality. We are interested in the behavior of Mn in the Ba4AMn3O12 family, and more broadly, in like-type Mn-based complex oxides. In these systems, the extent of electron sharing between Mn and the O ligand is highly malleable and greatly influences the material’s thermodynamic behavior. This can be seen by the stark differences between the capacity and hydrogen production yield in the Ba4AMn3O12 family upon substitution. In addition, the polytypes accessible to this family of compounds are distinguished by the complexity of the Mn-O oligomer rearrangements within the layered structures. Fundamentally, we strive to understand to what extent electronic structure and crystallographic arrangements influence the enhanced water splitting thermodynamics and kinetics we observe in these complex perovskites. We believe that a comprehensive understanding of local structure in such compounds will lead to novel pathways for modifying and improving redox activity in perovskites.